The present invention generally relates to an alloying process in a galvanization process and an alloying surface used to carry out the alloying process. More specifically, the invention relates to an alloying step performed subsequent to a step of dipping sheet steel into a molten zinc bath.
Conventionally, it has been well known to galvanize sheet iron to form a external Fe--Zn alloy layer in galvanized sheet iron production. In conventional alloying processes, it has been difficult to exert alloying heat uniformly over the entire surface of the sheet iron. As a result, in conventional galvanization processes including this Fe--Zn alloying step, the alloyed Fe--Zn layer tends to be unevenly alloyed. If the plating layer is alloyed with the iron of the sheet to an excessive degree, the tenacity of the plating layer is degraded and the galvanizing layer may peel or spall off during subsequent manufacturing processes, such as press-forming. Conversely, if the glavanizing Zn is not adequately alloyed with the iron, the plating layer will be too hard and may crack during later machining steps.
The uneven heating prevalent in conventional alloying techniques in due largely to the fact that the alloying furnace is positioned above a molten zinc bath through which the sheet iron is dipped for application of the zinc layer. The iron passes vertically through furnace from bottom to top. A plurality of burners arranged opposite the sheet iron path exert alloying heat on the zinc layer of the sheet iron as it passes through the furnace. The burners are arranged in an array extending both laterally and vertically in order to cover a broad area including the entire wide of the sheet iron and approximately the lower half of the furnace. This conventional arrangement of the burners within the furnace, however, tends to result in a locally uneven distribution of the burner fuel, such as natural gas, and/or the air supply. Uneven distribution of the fuel and/or air results in uneven combustion among the burners. This results in uneven heat distribution across the zinc-covered sheet iron and thus uneven alloying of the zinc layer. This may even directly subject the sheet iron to the burner flame, which would generate embrittled heat spots on the alloy surface.
As will be appreciated herefrom, heat distribution control is very important in the alloying process for galvanized sheet iron. In general, in order to obtain a high-quality Fe-Zn alloy layer on the surface of the sheet metal, heat of the alloying process must be applied uniformly over the entire surface of the sheet iron and within a temperature range of 600.degree. C. to 700.degree. C.
Furthermore, in order to achieve stable combustion in each burner, the length of the bore and so the thickness of the associated support tile must be sufficiently great. This results in increasing of total weight of the burner array. In the prior art, these relatively heavy burners were arrayed laterally and vertically, requiring relatively strong furnace walls to support them. This implied enlarged furnace walls made of refractory bricks. Such alloying furnaces are undesirably heavy and difficult to move from one molten zinc bath to another. In addition, since the furnace walls made of refractory bricks have a great heat capacity, the response characteristics to control adjustments of the furnace temperature are rather poor. Furnace temperature control is necessary for continuous treatment of sheet iron of differing thicknesses. In other words, in order to alloy sheet iron of a different thickness than the preceding sheet, the furnace temperature must be adjusted to ensure optimal alloying. However, since the thermal inertia of the furnace is so great, a substantial length of the new sheet will be badly alloyed.